Vitamin B-12 is a water-soluble vitamin from the B complex group, naturally found in foods like meat, fish, and dairy products. Vitamin B-12 injections can help prevent or treat pernicious anemia and B-12 deficiency. They may also boost your energy, metabolism, mood, and memory. If you're a vegetarian or dealing with fatigue, constipation, memory lapses, or low energy, you might be deficient in B-12, and supplementation could be a good option.